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Although most types of plastics can be repaired with adhesive materials, hot-air welding is usually preferred for thermoplastics because of its speed and ease. A plastic welder only takes a couple of minutes to heat up to operating temperatures and, once hot, can weld at speeds ranging from 5 to 30 inches per minute, depending on the application. Best of all one can go directly to and from welding to sanding and painting without waiting for the repair to cure (although it might be necessary to allow a few minutes for the welded plastic to cool).
Many adhesives do require mixing and can take anywhere from 30 minutes to several hours to cure, but heat can be used to shorten the curing time. This is not necessarily a disadvantage because repair technicians can always find something else to do while the adhesive cures. As for surface preparation, both welding and adhesives require some grinding, sanding, or trimming for good adhesion. More preparation is generally needed with adhesives, however, because the edges of the damaged area often have to be featheredged before the adhesive is applied. Some adhesives also require reinforcing or support patches behind the damaged area, making the repair a several step process.
To accomplish a plastic weld, either of two types of equipment can be used:
Hot-air welder
Airless
(Scharff R., Mullen K., Corinchock J.A., Complete Automotive Estimating, 1990, pg. 106)
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Plastic welding is a method of joining thermoplastics by one of sev-eral processes. These processes can be most generally classified as thermal-welding processes or solvent-welding processes.
By careful application of heat or solvent to a thermoplastic substrate, one may liquefy the surface resin and use it to form the bond. With thermal or solvent welding, surface preparation is not as critical as with adhesive bonding. The bond strength is determined by diffusion of polymer from one surface into another instead of by the wetting and adsorption of an adhesive layer. However, with welding some form of pretreatment may still be necessary. Certainly, the parts should be clean, and all mold release and contaminants must be removed by standard cleaning procedures.
Welding by application of heat or thermal welding provides an advantageous method of joining many thermoplastics that do not degrade rapidly at their melt temperature. It is a method of providing fast, relatively easy, and economical bonds that are generally 80-100 percent the strength of the parent plastic.
Thermal welding process can be of two kinds: direct and indirect. With direct welding, the heat is applied directly to the substrate in the form of either a heated tool or hot gas. Indirect heating occurs when some form of energy other than thermal is applied to the joint. The applied energy, which causes heating at the interface or in the plastic as whole, is generally in the form of friction, high-frequency electrical fields, electromagnetic fields, or ultrasonic vibration. Because the heating is local-ized at the bonding surface, indirect heating processes are very energy efficient, generally resulting in bonds that are stress free and of higher strength than those made by direct welding methods.
Solvent welding or cementing is the simplest and most economical method of joining many noncrystalline thermoplastics. Solvent-cemented joints are less sensi-tive to thermal cycling than joints bonded with adhesives, and they are as resistant to degrading environments as their parent plastic. Bond strength equaling 85-100 percent of the parent plastic can be obtained. The major disadvantage of solvent cementing is the possibility of stress cracking or crazing of the part and the possible hazards of using low-vapor-point solvents. When two dissimilar plastics are to be joined, adhesive bonding is generally desirable because of solvent and polymer compatibility problems.
Solvent cements should be chosen with approximately the same solubility para-meter as the plastic to be bonded. It is common to use a mixture of fast-drying solvent with a less volatile solvent to prevent crazing. The solvent cement can he bodied to 25 percent by weight with the parent plastic to fill gaps and reduce shrink-age and internal stress during cure.
The parts to be bonded should be unstressed and, if necessary, annealed. The sur-faces should mate well and have a clean, smooth surface. A V-joint or rounded butt joint is generally preferred for making a solvent butt joint. Scarf joints and fiat butt joints are difficult to position and to apply pressure to during the solvent evaporation phase of the process. The solvent cement is generally applied to the substrate with a syringe or brush. In some cases the surface can be immersed in the solvent. After the area to be bond-ed softens, the parts are mated and held under pressure until dry. Pressure should be low and uniform so that the finished joint will not be stressed. After the joint hard-ens the pressure is released, and an elevated temperature cure may be necessary, depending on the plastic and desired joint strength. The bonded part should not be packaged or stressed until the solvent has adequate time to escape from the joint.
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